No matter how many times it's said, I never tire of being reminded that writing is, above all else, a process. This in no way guarantees that everyone's process will ultimately pay off in publication, much less literary immortality, but it is a good reminder that these guys had drafts that looked like mine do now. And look at what we wouldn't have had if they had stopped short.
Check this out if you need some of that reassurance yourself. I especially like Marcel Proust's graphic, random, method of organization.